Control systems often include electrical isolation to partition the system into multiple power domains. In high voltage systems, isolation barriers isolate control and user interface circuits from dangerous power line voltages so as to block unwanted dangerous voltages across isolation domain barriers to prevent electrical shock to human operators and damage to electrical components, while permitting normal signal and power transfer between isolation domains. Example high voltage systems include industrial automation and control systems such as programmable logic controllers (PLC) and distributed control systems (DCS), inverters, motor drives, medical equipment, solar inverters, power supplies and hybrid electric vehicles (HEV).
A variety of isolation barriers are known, including the use of optical isolators that convert input electrical signals to light levels or pulses generated by light emitting diodes, and then receive and convert the light signals back into electrical signals. Isolators also exist which are based upon the use of Hall effect devices, magneto-resistive sensors, capacitive isolators and coil- or transformer-based (with core or coreless) isolators.
Isolation barriers are used to protect users by safely controlling the flow of power that is supplied from an ac supply to a load based in response to user commands. More specifically, for example, a typical motor drive system may include three power domains: command, control, and power. A safety constraint imposed upon the high voltage system is that the user command circuits must be galvanically isolated from dangerous voltages on the power circuit. In general, a determination is made as to whether to place an isolation barrier between the command and the control circuits or to place an isolation barrier between the control and the power circuits.
FIG. 1 is an illustrative schematic diagram of a motor control system 102 showing electrical isolation between a live power domain 104 and safety-earth power domain 106. Electrical separation of the two power domains is represented by the dashed line 107. The live power domain 104 includes AC voltage supply 108, AC-to-DC converter 109, drive stage 110 and motor 112. The drive stage 110 includes multiple Insulated-Gate Bipolar Transistors (IGBTs) 114 or power MOSFet (Metal Oxide Semiconductor Field effect transistor) configured to convert a DC voltage to a provide a multi-phase, typically three-phase AC drive current provided to the motor 112 coupled as shown. The safety-earth power domain 106 includes control circuit 116 and communication circuits 118. The control circuit 116 produces signals to control operation of the drive stage 110 in response to feedback signals produced by the drive stage 110. The communication circuit 118, which may include local user interface controls such as keyboard and mouse (not shown) or remote control signals through a bus system (not shown), for example, receives user input commands for delivery to the control circuit 116. First control lines 120 are coupled to communicate drive control signals from the control circuit 116 to the drive stage 110. Second control lines 122 are coupled to communicate current feedback control signals from the output of the drive stage 110 to the control circuit 116. Third control lines 123 are coupled to communicate user input commands from the communication circuit 118 to the control circuit 116. First isolation circuits 124 coupled to the first control lines 120 impose a first electrical isolation barrier between the control circuit 116 and the drive stage 110. Second isolation circuits 126 coupled to the second control lines 122 impose a first electrical isolation barrier between the control circuit 116 and output of the drive stage 110. The first isolation circuit includes a separate transformer coupled to each one of the first lines 120 to separate it into two electrically isolated line segments. The second isolation circuit 122 includes a separate transformer in combination with a separate ADC modulator coupled to each one of the second lines 122 to separate it into two electrically isolated line segments.
Different levels of isolation may be provided between different power domains. Systems typically comply with safety requirements defined by international standards such as IEC 61800, IEC 61508 and IEC 62109 that cover applications like motor drives and solar inverters. Safety standards such as International Standard IEC 60950-1 and IEC 60747-17 specify several different electrical isolation levels. Functional isolation is used to enable system components to transmit and receive signals between them while maintaining signal integrity and amplification so that they can function properly. A functional isolation barrier typically does not protect a user from electrical shock. Basic isolation provides an additional second level of isolation to protect from electrical shock. Double isolation provides an additional level of isolation for safety reasons, i.e. twice the basic isolation. Reinforced isolation provides even greater protection from high voltages.
Placement of an isolation barrier between circuits can result in degraded signal integrity, added cost and increase in isolation barrier bulk, requiring additional physical space. As a consequence, a tradeoff often is necessary between the number signal lanes provided between electrical components within different power domains of a system and the level electrical isolation provided between them.
Boundaries between command, control, and power domains are sometimes blurred due to a recent trend to integrate more functions into fewer physically separate components. For example, certain control functions and command functions often are integrated within a common processor device. As a result, fewer physical power domain boundaries may exist within a system at which to locate isolation barriers.